Additional information is disclosed in the following documents: International Patent Publication WO 2010/120581 A1, published Oct. 21, 2010, for “Method for Producing Solid Carbon by Reducing Carbon Oxides;” International Patent Publication WO 2013/158156, published Oct. 24, 2013, for “Methods and Structures for Reducing Carbon Oxides with Non-Ferrous Catalysts;” International Patent Publication WO 2013/158159, published Oct. 24, 2013, for “Methods and Systems for Thermal Energy Recovery from Production of Solid Carbon Materials by Reducing Carbon Oxides;” International Patent Publication WO 2013/158160, published Oct. 24, 2013, for “Methods for Producing Solid Carbon by Reducing Carbon Dioxide;” International Patent Publication WO 2013/158157, published Oct. 24, 2013, for “Methods and Reactors for Producing Solid Carbon Nanotubes, Solid Carbon Clusters, and Forests;” International Patent Publication WO 2013/158158, published Oct. 24, 2013, for “Methods for Treating an Offgas Containing Carbon Oxides;” International Patent Publication WO 2013/158155, published Oct. 24, 2013, for “Methods for Using Metal Catalysts in Carbon Oxide Catalytic Converters;” International Patent Publication WO 2013/158161, published Oct. 24, 2013, for “Methods and Systems for Capturing and Sequestering Carbon and for Reducing the Mass of Carbon Oxides in a Waste Gas Stream;” International Patent Publication WO 2014/011206, published Jan. 16, 2014, for “Methods and Systems for Forming Ammonia and Solid Carbon Products;” and International Patent Publication WO 2013/162650, published Oct. 31, 2013, “Carbon Nanotubes Having a Bimodal Size Distribution.” The entire contents of each of these documents are incorporated herein by this reference.
Solid carbon has numerous commercial applications. These applications include longstanding uses such as uses of carbon black and carbon fibers as a filler material in tires, inks, etc., many uses for various forms of graphite (e.g., pyrolytic graphite in heat shields) and innovative and emerging applications for buckminsterfullerene and carbon nanotubes. Conventional methods for the manufacture of various forms of solid carbon typically involve the pyrolysis of hydrocarbons in the presence of a suitable catalyst. Hydrocarbons are typically used as the carbon source due to historically abundant availability and relatively low cost. The use of carbon oxides as the carbon source in the production of solid carbon has largely been unexploited.
Carbon oxides, particularly carbon dioxide, are abundant gases that may be extracted from point-source emissions such as well gases, the exhaust gases of hydrocarbon combustion or from some process offgases. Carbon dioxide may also be extracted from the air. Because point-source emissions have much higher concentrations of carbon dioxide than does air, they are often economical sources from which to harvest carbon dioxide. However, the immediate availability of air may provide cost offsets by eliminating transportation costs through local manufacturing of solid carbon products from carbon dioxide in air.
Carbon dioxide is increasingly available and inexpensive as a byproduct of power generation and chemical processes in which an object may be to reduce or eliminate the emission of carbon dioxide into the atmosphere by capture and subsequent sequestration of the carbon dioxide (e.g., by injection into a geological formation). For example, the capture and sequestration of carbon dioxide is the basis for some “green” coal-fired power stations. In current practice, capture and sequestration of the carbon dioxide entails significant cost.
There is a spectrum of reactions involving carbon, oxygen, and hydrogen wherein various equilibria have been identified. Hydrocarbon pyrolysis involves equilibria between hydrogen and carbon that favors solid carbon production, typically with little or no oxygen present. The Boudouard reaction, also called the “carbon monoxide disproportionation reaction,” is the range of equilibria between carbon and oxygen that favors solid carbon production, typically with little or no hydrogen present. The Bosch reaction is within a region of equilibria where all of carbon, oxygen, and hydrogen are present under reaction conditions that also favor solid carbon production.
The relationship between the hydrocarbon pyrolysis, Boudouard, and Bosch reactions may be understood in terms of a C—H—O equilibrium diagram, as shown in FIG. 1. The C—H—O equilibrium diagram of FIG. 1 shows various known routes to solid carbon, including carbon nanotubes (“CNTs”). The hydrocarbon pyrolysis reactions occur on the equilibrium line that connects H and C and in the region near the left edge of the triangle to the upper left of the dashed lines. Two dashed lines are shown because the transition between the pyrolysis zone and the Bosch reaction zone may change with reactor temperature. The Boudouard, or carbon monoxide disproportionation reactions, occur near the equilibrium line that connects O and C (i.e., the right edge of the triangle). The equilibrium lines for various temperatures that traverse the diagram show the approximate regions in which solid carbon will form. For each temperature, solid carbon may form in the regions above the associated equilibrium line, but will not generally form in the regions below the equilibrium line. The Boudouard reaction zone appears at the right side of the triangle. In this zone, the Boudouard reaction is thermodynamically preferred over the Bosch reaction. In the region between the pyrolysis zone and the Boudouard reaction zone and above a particular reaction temperature curve, the Bosch reaction is thermodynamically preferred over the Boudouard reaction.
CNTs are valuable because of their unique material properties, including strength, current-carrying capacity, and thermal and electrical conductivity. Current bulk use of CNTs includes use as an additive to resins in the manufacture of composites. Research and development on the applications of CNTs is very active with a wide variety of applications in use or under consideration. One obstacle to widespread use of CNTs has been the cost of manufacture.
U.S. Pat. No. 7,794,690 (Abatzoglou, et al.) teaches a dry reforming process for sequestration of carbon from an organic material. Abatzoglou discloses a process utilizing a 2D carbon sequestration catalyst with, optionally, a 3D dry reforming catalyst. For example, Abatzoglou discloses a two-stage process for dry reformation of an organic material (e.g., methane, ethanol) and CO2 over a 3D catalyst to form syngas, in a first stage, followed by carbon sequestration of syngas over a 2D carbon steel catalyst to form CNTs and carbon nanofilaments. The 2D catalyst may be an active metal (e.g., Ni, Rh, Ru, Cu—Ni, Sn—Ni) on a nonporous metallic or ceramic support, or an iron-based catalyst (e.g., steel), on a monolith support. The 3D catalyst may be of similar composition, or may be a composite catalyst (e.g., Ni/ZrO2—Al2O3) over a similar support. Abatzoglou teaches preactivation of a 2D catalyst by passing an inert gas stream over a surface of the catalyst at a temperature beyond its eutectic point, to transform the iron into its alpha phase. Abatzoglou teaches minimizing water in the two-stage process or introducing water in low concentrations (0 to 10 wt %) in a reactant gas mixture during the dry reformation first stage.